Abstract
The objective of this study was to evaluate the efficiency of gonadotropin releasing hormone (GnRH) and GnRH doses in synchronizing follicular wave emergence as a pretreatment for superovulation in cattle. Fourteen Holstein-Friesian cows 6 days from estrus were randomly assigned to receive 100 μg (n=4), 50 μg (n=5), or 25 μg (n=5) of GnRH. Superovulation was induced with injections of porcine FSH (pFSH) twice daily, decreasing the dose (total 42 AU) over five days begining 2.5 days after receiving GnRH. On the 7th and 8th injections of pFSH, 750 μg of PGF2a was also given. With the exception of one cow that was given 50 μg of GnRH, ovulation was induced in all cows from the three groups and the new follicular wave emergence was observed. The total number of follicles for the 25 μg GnRH group was less than that observed for the 100 μg GnRH group (P<0.05), although there were no differences between the 100 μg, 50 μg and 25 μg GnRH groups with respect to the number of preovulatory follicles (≥10 mm) and CL. The numbers of normal embryos were greater for the 25 μg GnRH group than the 100 or 50 μg GnRH groups (P<0.01); however, the numbers of ova/embryos did not differ significantly between the three groups. These results suggest that 25 μg of GnRH was sufficient to induce ovulation and follicular wave emergence. On day 6 of the estrous cycle, a reduction of the dose of GnRH to synchronize follicular wave emergence as a pretreatment for superstimulation promotes transferable embryos.
